1. Field of the Invention
The invention relates to a method for forming films, and in particular to a method for improving the uniformity of wafer-to-wafer film thicknesses used in a plasma-enhanced chemical vapor deposition (PECVD) reactor.
2. Description of the Related Art
A PECVD system includes a reactor, a gas transmission system, an exhaust system, and a process control system. This specification focuses on the reactor while the other systems not related to the invention are not further described. The Novellus PECVD system is taken as an example, and the reactor system thereof is shown in FIG. 1. In FIG. 1, an RF power source 12 is applied to 6 shower heads 14 in a reactor 10, and 6 wafers 16 is placed on a heater block 18, such that the wafers 16 is opposite to the shower head 14.
The PECVD system is typically used to deposit dielectric films. In semiconductor technology, two metal layers are isolated by a dielectric layer for the purpose of insulation. Therefore, the property, quality and thickness of the dielectric layer are significant considerations.
As an example, a tetra-ethyl-ortho-silicate (TEOS) is traditionally used to serve as a dielectric layer for isolating metal layers. In the Novellus PECVD system, a TEOS layer is deposited at a high temperature of 300-400.degree. C. Furthermore, the shower head 14 does not have any temperature control device mounted on it and the thicknesses of TEOS layers are determined only by setting a deposition time. Therefore, the unstable temperature of the shower heads causes the TEOS layers to be deposited with varying thicknesses on the first several wafers. The temperature of the shower heads 14 stabilizes only after a certain number of wafers have been subjected to a deposition process. Once the temperature stabilizes, the thicknesses of the TEOS layers can also reach a uniform value. Referring to FIG. 2, the y-axis indicates the thicknesses of TEOS layers or the temperature of the shower heads while the x-axis indicates the number of wafers on which the deposition process has been completed.
Therefore, when TEOS layers are deposited by using a prior PECVD system, the uniformity of the thicknesses of the TEOS layers varies from batches to batches. The obvious difference in thickness among the wafers makes the quality of products very difficult to control, resulting in poor yield.
Moreover, after the PECVD system operates for a period of time, it is necessary to perform preventive maintenance or open chamber clean for the system to continue to work properly. However, it takes time to heat the heater block to production temperature, increasing the time required to start the PECVD system after implementing preventive maintenance or cleaning.